1. Field of the Invention
The present invention relates generally to rocket thrust chambers and more particularly to fixed geometry rocket thrust chambers having means for varying their nozzle expansion ratios.
2. The Prior Art
In the past, it was almost universal practice to use staged launch vehicles for orbital or extra-orbital space missions such that the engines of each stage could be equipped with nozzles having expansion ratios befitting the prevailing ambient pressure conditions and performance requirements for the particular stage. Since first stage engines had to be operable at sea level ambient conditions, they were commonly equipped with nozzles having a moderate expansion ratio, whereas those of the upper stages were commonly equipped with nozzles having much higher expansion ratios.
With today's unstaged launch systems, problems arise as to how to optimalize the engine(s) of the launch vehicle so that it can operate efficiently both at the launch pad and in-space. The problem is two-fold in that an engine whose expansion ratio is optimal at launch is grossly underexpansive in space. Secondly, an engine which is sized for launch will be too big for use in space and will overaccelerate the space vehicle and/or its occupants if it is operated at full thrust in the latter phases of the launch. So far, the choice has been to use fixed geometry engines which have intermediate expansion ratios; this solution causes the engines to suffer performance penalties from overexpansion at launch and from under-expansion at all other phases of their mission.
Of course, practitioners of the prior art have attempted to resolve the above problems by "unfixing" the geometry of the nozzle by means of using deployable nozzle sections or inflatable, extendable nozzle plugs or other mechanical means for changing the contour of the nozzle, the idea being that once the rocket motor reaches space, the geometry of its nozzle could be changed to give the desired greater nozzle ratio. However, these devices add great weight and complication to the rocket motor, and are subject to failure because of the hostility of their working environment.